Richard Linklater's "Me and Orson Welles" is one of the sweetest and most heartfelt movies ever made about a life in the theater. This should come as no surprise to anyone who has closely followed Linklater's career, which encompasses everything from "School of Rock" to "Waking Life" to the great young-love duet, "Before Sunrise" and "Before Sunset." He has both a populist's touch and a humanist's eye. It's a great and rare combination, and it serves him particularly well in this movie about a theater-struck high school teenager unceremoniously ushered into the fabulous world of that sacred monster, Orson Welles.

Quite by chance, Richard Samuels (Zac Efron) is cast in the bit role of Lucius in Welles's daring adaptation of "Julius Caesar," which is in its final week of rehearsal. (The actors are uniformed as Italian Fascists.) He enters into a world within a world where emotions run as high offstage as on and everyone is in fearful awe of the 22-year-old boy genius (Christian McKay).

Linklater and his screenwriters, Holly Gent Palmo and Vince Palmo, adapting the novel by Robert Kaplow, showcase the wraparound tumult of putting on a production, and they do it as if this sort of thing had never been filmed before. When Linklater made his "Sunrise/Sunset" films, the first stirrings of love seemed to be taking place right before our eyes. Similarly incandescent, "Me and Orson Welles" showcases an ardor for theater – for life lived at its highest pitch.

For Richard, the theater is also his entrance into a more earthly infatuation. Welles's all-purpose assistant Sonja (Claire Danes) is lusted after by most of the troupe's actors, who also include Norman Lloyd (Leo Bill) and Joseph Cotten (James Tupper). Bemused by his innocence, she leads Richard on. Gaga from her attentions, he fancies the infatuation runs both ways. What he doesn't recognize is Sonja's all-purpose drive to get ahead. When he discovers her relationship with Welles is more than all-business, she explains pragmatically, "I have to take care of myself," and the words hit him like slaps.

Efron has the sleek, retro look here of a 1930s matinee idol, a young Tyrone Power perhaps. He's charming. The big splash in the cast, though, comes from McKay's Welles. With the exception of Philip Seymour Hoffman's Truman Capote in "Capote," I have never seen a famous-person performance this accomplished. It's not just that McKay, a British actor who has performed as Welles on stage, looks and sounds uncannily like the real deal. He gives us Welles as a fully formed creation – an enfant terrible with the wiles and mores of an aging roué. This genius is a credit-hogging behemoth whose instinct for the right theatrical effect is as unerring in real life as on the stage (for Welles, the distinction may be moot). Linklater makes you feel exactly as Welles's Mercury Theater players did: They may cower before him and curse him behind his back, but they know that this is the experience of a lifetime. They feel bludgeoned and anointed at the same time.

Welles in this film is so larger than life that, for a while, I was afraid he might become a roaring caricature. But Linklater gives Welles a beautiful, brief sequence where, riding with Richard en route to a radio show taping, he pulls out a marked copy of Booth Tarkington's novel "The Magnificent Ambersons" and drops his guard for a moment. The book, he ventures, "is about how everything gets taken away from you," and the moment is extraordinarily moving not only because we know that Welles years later will direct the film of "The Magnificent Ambersons" (which the studio took away from him and recut). It's moving, and also creepy, because this prodigious young man resounds with a sense of loss he has yet to fully experience in his own life. We think, too, of the losses in his movie career as it unfolded, the botched and unfinished projects. We think of Welles's legendary self-destructiveness that, here, in nascent form, is already gathering force.

But all thoughts of impending gloom are momentarily stayed on opening night, when Welles's production of "Julius Caesar" gets a standing ovation. He mutters to himself, "How the hell do I top this?" The glint in his eye tells us he's not worried in the slightest. Richard, meanwhile, cast off by Welles, remains enthralled. This teenager has just experienced something much bigger than himself. He speaks in the end about how all of life seems to be ahead of him, and you can't help but share in his rapture. Grade: A (Rated PG-13 for sexual references and smoking.)